The observed behaviour of Notomy alexis (Spinifex hopping mice) fits poorly into predictive models of mammalian social organization and breeding systems when these models are based on aspects of general and reproductive morphology. A number of peculiarities in their reproductive physiology are indicative of on-gregarious lifestyles and monogamy. In particular, relative to body size, males of the species have testis mass and volume an order of magnitude smaller than most other mammals. In captivity, however, Notomys alexis demonstrate polyandry - a mating system that tends to result in sperm competition.